One of the biggest problems that determine the limitation in performance of supercomputers in recent years is power consumption, and the importance of researches relating to the power-saving capability of supercomputers has already been recognized widely. That is, the speed performance per consumed power (Flops/W) has become one barometer for evaluating the supercomputers. Further, in data centers, it is understood that 45% or so of the power consumption by the whole data centers are consumed for cooling, and therefore, a demand for reduction of the power consumption through improvements in cooling efficiency has become strong.
Heretofore, an air-cooling type and a liquid-cooling type have been in use for cooling supercomputers and data centers. The liquid-cooling type is generally recognized to be high in cooling efficiency because of using a liquid that is remarkably superior to air in heat transfer performance. For example, the “TSUBAME-KFC” built by Tokyo Institute of Technology achieved 4.50G Flops/W by a liquid immersion cooling system using a synthetic oil and acquired the first place in “Supercomputer Green 500 List” announced on Nov. 2013 and Jun. 2014. However, because the synthetic oil being high in viscosity is used as the cooling liquid, it is difficult to completely remove, from electronic devices taken out from oil-immersed racks, the oil adhered thereto, and this gives rise to a problem that the maintenance (specifically, adjustment, inspection, repair, replacement and expansion, for example; the same applies hereafter) of the electronic devices is extremely difficult. Furthermore, the occurrence of a problem has also been reported that causes a difficulty to arise in practical use because the synthetic oil in use leaks by corroding a gasket and the like constituting the cooling system in a short period of time.
On the other hand, there has been proposed a liquid immersion cooling system that uses not the synthetic oil causing the aforementioned problems but a cooling liquid of fluorocarbon-base. Specifically, it is an example that uses a cooling liquid of the fluorocarbon-base (a hydrofluoroether (HFE) compound known as “Novec (trademark of 3M Company; the same applies hereafter) 7100”, “Novec 7200” and “Novec 7300”, brand names of 3M Company) (Patent Literature 1 and Patent Literature 2, for example).
By the way, in order to locally cool a heating element such as a CPU that generates heat of a particularly large amount, a number of proposals have been made regarding the examples of cooling systems using a boiling cooling device that performs heat transfer and radiation through the cycle of vaporization and condensation of a cooling liquid. One is an example of a cooling module wherein a vaporization section connected to a heating surface of a processor and a condensation section connected to a cooling fan or a water cooling pipe are connected through two pipes to perform refrigerant circulation utilizing vapor-liquid equilibrium (Non Patent Literature 1). Another is an example wherein a cooling liquid is enclosed in a tabular receptacle with a special flow passage wall defined inside, wherein a heat receiving region of the tabular receptacle is thermally connected to a heating element while a heat radiation region of the tabular receptacle is connected to a heat radiation section such as a radiating fin or the like, and wherein the heat radiation region defines a flow passage for a cooling liquid in the heat radiation region (Patent Literature 3 , for example).